ACTG 254 is a comparative study of two combination drug therapies for the prevention of serious bacterial infections in HIV-infected children between 2 and 18 years of age. Drug combinations being evaluated are trimethoprim-sulfamethoxazole versus atovaquone plus azithromycin. Nationwide, approximately 300 patients have been enrolled (target enrollment = 670). CHOP has enrolled 5 patients into this study (one within the past year). An intensive pharmacokinetic substudy (ACTG 954) was added in April 1996 to evaluate a potential interaction between atovaquone and azithromycin. Enrollment into this substudy is complete (no patients were enrolled in this substudy at CHOP). No preliminary data are yet available from the main study or the substudy. Our plan is to continue to screen patients for eligibility to enter this study.